Waterproof & Water Repellent Finishing in Textiles
How is a Water Repellent Finish Applied in Textiles?
Water repellent finish is applied to textiles through a three-step wax and metal salt treatment process: first, the fabric is impregnated in a soluble sodium soap bath; second, it is treated with a polyvalent metal salt such as aluminium sulfate that precipitates the hydrophobic soap onto the fabric surface, anchoring the repellent finish; and third, a combination of crease-resistant resin with paraffin is applied to improve durability through repeated wash cycles. This traditional wax-metal salt method, combined with modern silicone-based finishes, provides durable water repellency while retaining fabric breathability.
The two primary water-repellent finish methods each offer distinct characteristics: wax-metal salt finishes use hydrophobic soap precipitation and provide excellent water beading, while silicone (polydimethylsiloxane) finishes deliver a softer fabric handle with good durability. Both methods create a hydrophobic surface that causes water to bead and roll off rather than soaking through the fabric structure.
Three terms define fabric water performance — water-resistant fabrics withstand light exposure but fail under prolonged or heavy water pressure; water-repellent fabrics are treated with a hydrophobic finish that causes water to bead and roll off while retaining air permeability; and waterproof fabrics are impervious to water under pressure and meet ASTM D3393 requirements. The table below provides full specifications:
| Term | Definition | Water Column Resistance |
|---|---|---|
| Water-resistant | Fabric can withstand light water exposure; however, prolonged or heavy exposure causes water to penetrate through the fabric structure. | 500–1,000 mm (ISO 811) |
| Water-repellent | Fabric is treated with a hydrophobic finish that causes water to bead and roll off the surface rather than soaking in. Air permeability is retained. | 1,000–5,000 mm |
| Waterproof | Fabric is impervious to water under pressure. Meets or exceeds ASTM D3393 requirements of 1,000 mm hydrostatic resistance (9.8 kPa). | 5,000–20,000+ mm |
The Water Repellent Finish Process Uses Wax and Metal Salt Treatment
The surface of a hydrophilic fabric is treated with wax materials such as paraffin or hydrophobic insoluble soap compounds such as sodium stearate to make it water-repellent. The three-step process is:
First, the fabric is impregnated in a bath containing a soluble sodium soap solution.
Then, the fabric is impregnated in a second bath containing a polyvalent metal salt (such as aluminium sulfate or chrome alum) that precipitates the hydrophobic soap onto the fabric surface, anchoring the repellent finish.
A combination of crease-resistant resin with paraffin is applied to improve the permanency of the finish through repeated washing cycles.
If the emulsion of paraffin contains an anionic emulsifying agent, the addition of a polyvalent cation such as Al3+ prevents the repulsion between the anionic emulsion and cotton’s negatively charged surface, ensuring even distribution of the repellent agent.
Water Repellent Fabrics Retain Air Permeability
Water-repellency is provided by a hydrophobic film on the surface of the fabric. The interstices between yarn bundles remain unfilled, so water vapor and air can still pass through. This means water-repellent fabrics retain a degree of air permeability that fully waterproof fabrics do not.
Requirements for Introducing Water Repellency
Silicone Finish Delivers Soft Handle and Water Repellency
Modern water-repellent fabric finishes widely use an emulsion of polydimethylsiloxanes (PDMS), commonly referred to as silicone finish. Polysiloxanes (silicones) are available in liquid, rubber, and solid forms, but the liquid emulsion form is preferred for textile application due to ease of coating.
The key advantages of silicone finishes are their rapid spreading behavior on fabric surfaces and the softer handle they impart alongside durable water-repellency. The finish forms a flexible, hydrophobic polymer network that remains bonded to the fiber surface through multiple wash cycles.
Partially polymerized methylsiloxanes in an organic solvent or aqueous dispersion are the primary formulations used in textile finishing. The degree of water-repellency depends directly on the orientation of the polysiloxane molecules on the fiber surface — methyl groups orient outward away from the polar fiber, while oxygen atoms face toward the fiber.
The incorporation of zirconium or titanium compounds cross-links the silicone polymer network and orients an array of hydrophobic methyl groups outward from the fiber surface, delivering durable water-repellency with a notably softer fabric handle compared to traditional wax-metal salt finishes.
Requirements for Applying Silicone Finish
The fabric must be free from surface-active wetting agents to ensure good adhesion of the silicone polymer. Thorough rinsing of the fabric after previous processing is essential before silicone application.
Wax-Metal Salt vs Silicone Finish: Method Comparison
| Property | Wax-Metal Salt Method | Silicone (PDMS) Method |
|---|---|---|
| Chemical basis | Paraffin wax + sodium stearate + metal salt (aluminium sulfate or chrome alum) | Polydimethylsiloxane (PDMS) emulsion |
| Application method | 3-step bath impregnation process | Single-stage padding or exhaust application |
| Fabric handle | Firm to stiff hand feel | Soft, flexible handle retained |
| Durability | Good wash durability with resin addition; can stiffen after repeated laundering | Excellent wash durability; cross-linking with Zr/Ti compounds enhances permanence |
| Air permeability | Fully retained (interstices remain unfilled) | Fully retained |
| Water beading | Excellent immediate beading | Good beading with softer surface feel |
| Compatibility | Best on cotton, cotton blends; requires removal of surface-active agents | Broad fiber compatibility including synthetics; requires clean fabric surface |
| Cost | Lower raw material cost | Higher raw material cost |
Waterproof fabrics are impermeable to water, yet they must also allow breathability for wearer comfort. Waterproof breathable fabrics are classified as technical textiles designed for protection against rain, wind, and body heat loss while managing moisture vapor from perspiration.
Early waterproof fabrics were uncomfortable to wear because they were stiff and prevented the escape of perspiration vapor. Modern waterproof breathable fabrics allow water vapor to passively diffuse through the fabric while blocking liquid water, maintaining comfortable body temperature during physical activity.
There are three primary categories of waterproof breathable fabric technology:
Construction of Waterproof Fabric
Waterproof garments combine multiple functional layers to achieve both waterproofing and wearer comfort. The standard construction includes three distinct layers:
- The face fabric — typically nylon or polyester, often treated with a DWR (Durable Water Repellent) coating of expanded PTFE. This layer provides external protection, abrasion resistance, and aesthetic properties.
- The membrane — the waterproof barrier layer, commonly polyurethane or ePTFE. This is the functional waterproof element that prevents liquid water penetration while allowing moisture vapor to pass through.
- A scrim or mesh lining — a comfort layer bonded to the inner surface. Full 3-layer constructions integrate this lining. In 2-layer or 2.5-layer garments, a separate loose-hanging lining provides this function.
Requirements for Producing Waterproof Fabric
- All seams must be sealed with waterproof tape heat-applied over the seam joins. This prevents water ingress through needle-punched seam holes. The tape uses a polyurethane adhesive that bonds to the seam under heat and pressure.
Two types of seam sealing are used in waterproof garment construction:
- Fully sealed garments — all seams are tape-sealed, used in extreme wet conditions such as marine, mountaineering, and professional outdoor workwear.
- Critically sealed garments — only high-stress seams (neck, chest, and shoulder areas) receive tape sealing. Body side seams and sleeve seams may be left unsealed to reduce cost and improve ventilation in moderate conditions.
- In the Durable Water Repellent (DWR) method, the fabric surface is treated so that water beads up and rolls off rather than soaking into the fiber. This surface treatment is critical for maintaining the outer fabric’s performance and preventing “wetting out” that blocks the breathable membrane beneath.
Waterproof Ratings Using the Static Column Test Method
The most widely recognized method for measuring waterproof performance is the static column test (hydrostatic head test). A 1-inch × 1-inch (25 mm × 25 mm) square tube is placed vertically against a fabric sample, and water is added to the tube. The test records the water column height in millimetres (mm) that the fabric can withstand without leaking through.
Industry benchmarks for hydrostatic resistance ratings are:
References
- Horrocks, A.R. and Anand, S.C. (eds.). Handbook of Technical Textiles. 2nd ed. Woodhead Publishing. ISBN: 978-1-78242-465-9.
- Broadbent, A.D. Basic Principles of Textile Coloration. Society of Dyers and Colourists. ISBN: 978-1-9017-1954-1.
- ASTM International. ASTM D3393-91(2022): Standard Specification for Coated Fabrics — Waterproofness. ASTM Book of Standards, Volume 09.02.
- Wikipedia. (2024). Waterproof Fabric. Wikimedia Foundation.
